There is known a plastic deformation tapping tool with inside diameter finishing edges, (a) comprising an external thread portion provided with a complete thread section, and a chamfered end section which is formed contiguously to the complete thread section and a diameter of which decreases in its axial direction toward its end face, and wherein protruding parts and relief parts are alternately formed in at least a length portion of the tapping tool from the chamfered end section to a first turn of thread of above-described complete thread section, so that the protruding parts are successively forced into an inner circumferential surface of a hole to be tapped, to form an internal thread in the inner circumferential surface by plastic deformation, (b) and wherein a flute is formed axially so as to divide a crest of the above-described external thread portion, in parallel with an axis O through the above-described chamfered end section and the above-described complete thread section, and the complete thread section is formed with the inside diameter finishing edges partially defined by an inner wall surface of the flute functioning as a rake surface, so that the inside diameter finishing edges remove by cutting an apex of a crest of the above-described internal thread, which crest is formed by the above-indicated plastic deformation (as disclosed in Patent Document 1).
A tapping tool 100 shown in FIGS. 5 is an example of such a conventional plastic deformation tapping tool with inside diameter finishing edges. FIG. 5(a) is a front elevational view taken along arrows VA in FIGS. 5(b) and 5(c), which are perpendicular to axis O, and FIGS. 5(b) and 5(c) are enlarged cross sectional views taken along respective lines VB-VB and VC-VC in FIG. 5(a). This plastic deformation tapping tool 100 is integrally provided with: a shank 12 at which the tapping tool 100 is attached to a main spindle through a chuck not shown; a neck portion 14 having a smaller diameter than the shank 12; and an external thread portion 16 having an external thread for forming an internal thread by plastic deformation (by rolling operation), such that the shank 12, neck portion 14 and external thread portion 16 are arranged in the order of description, coaxially with the axis O.
The above-described external thread portion 16 has a crest 18 which has a cross sectional shape corresponding to a shape of a root of the internal thread to be formed, and which is formed so as to extend along a helix having a lead angle of the internal thread. The external thread portion 16 is provided with a complete thread section 26 having a diameter substantially constant in its axial direction, and a chamfered end section 24 a diameter of which decreases in its axial direction toward its end face. Over entire lengths of the chamfered end section 24 and complete thread section 26 of the external thread portion 16, the crest 18 has six radially outwardly extending protruding parts 20a-20f, and relief parts 22 formed contiguously to and having smaller diameters than the respective protruding parts 20a-20f, such that the protruding parts 20a-20f and the relief parts 22 are arranged alternately in a direction of formation of an external thread of the external thread portion 16, with an angular spacing pitch of 60° about the axis O. Thus, the external thread portion 16 takes the form of a substantially regular hexagonal prism. Namely, the protruding parts 20a-20f which define respective six apexes of a regular or equilateral hexagon constitute respective six linear arrays each of which consists of a multiplicity protruding parts 20 successively arranged in a direction parallel to the axis O and which are equally angularly spaced apart from each other about the axis O. It is noted that FIGS. 5(b) and 5(c) are both cross sectional views taken along the helix of the root of the crest 18.
The above-described external thread portion 16 is formed by a grinding operation with a thread grinding member 30 (shown in FIG. 2) which has an outer circumferential grinding surface with a cross sectional shape corresponding to that of the crest 18. The diameters of the above-described protruding parts 20a-20f, relief parts 22 and chamfered end section 24 are adjusted to respective nominal values by moving the thread grinding member 30 toward and away from a workpiece used to form the tapping tool. That is, the crest 18 has a constant cross sectional shape over the entire lengths of the chamfered end section 24 and complete thread section 26, but the protruding parts 20a-20f, relief parts 22 and chamfered end section 24 have respective different outside diameters, respective different effective diameters and respective different root diameters. Broken lines in FIGS. 5(b) and 5(c) indicate the crest 18 formed by the above-indicated grinding operation, and the above-indicated regular hexagonal prism is the form of the workpiece immediately after the crest 18 is formed by the grinding operation.
Flutes 28 are formed adjacent to the apex ridges of the respective three arrays of the protruding parts 20a, 20c and 20e which are spaced apart from each other about the axis O by the other protruding parts of the six arrays of the protruding parts 20a-20f. The flutes 28 are formed on the downstream side of the three arrays of the protruding parts 20a, 20c, 20e as seen in a tap rotating direction (rotating direction of the tapping tool) A, and have a predetermined width dimension, to allow a lubricating fluid to be supplied to the end face of the tapping tool during its tapping operation. In the complete thread section 26, the three arrays of the protruding parts 20a, 20c, 20e function as large-diameter protruding parts which have a larger diameter than the other three arrays of the protruding parts 20b, 20d, 20f, and which are provided for forming inner circumference finishing edges 34. That is, the portion of each large-diameter protruding part which remains after the flute 28 is formed and which exists on the side remote from the flute 28 as seen circumferentially about the axis O is removed by the grinding operation, down to the root of the crest 18, so as to form a substantially flat relief surface 32, so that the inner circumference finishing edge 34 is formed along one of opposite open-end edges of the flute 28, which one open-end edge is on the upstream side of the flute 28 as seen in the tap rotating direction A. The inner circumference finishing edges 34 have the constant diameter not only in the complete thread section 26 but also in the chamfered end section 24. In the chamfered end section 24 in which the protruding parts 20a-20f have diameters which increase in steps, the portions of the three arrays of the protruding parts 20a, 20c, 20e which exist on the downstream side of their apex ridges as seen in the tap rotating direction A are removed by formation of the flutes 28, so that these protruding parts 20a, 20c, 20e are not forced into the inner circumferential surface of the hole to be tapped, whereby these protruding parts 20a, 20c, 20e are not able to cause plastic deformation of the inner circumferential surface. For this reason, the relief surfaces 32 are formed to provide the inside diameter finishing edges 34 also in the chamfered end section 24, to prevent the otherwise remaining crest 18 other than the flute region in the protruding parts 20a, 20c, 20e from interfering the hole. Patent Document 1 discloses hole clean-up edges having a larger diameter than the inside diameter finishing edges 34.